The objective of this study is to determine an adult equivalent dose of ABT-378/ritonavir in HIV-infected children based on the pharmacokinetics and tolerability of ABT-378/ritonavir in combination with reverse transcriptase inhibitors. The safety and antiviral activity of ABT-378/ritonavir in combination with reverse transcriptase inhibitors will be characterized. One hundred (100) subjects will participate in the study M98-940. These subjects will be stratified according to age at enrollment (i.e., three months to two years and two years to twelve years) and prior antiretroviral experience (i.e., nanve and non-nanve) prior to randomization. On study Day -1, all subjects will be equally randomized to one of two open-label dosing arms according to a previously generated randomization schedule. Fifty (50) subjects will receive ABT-378/ritonavir at a dose of 230/57.5 mg/m2 Q12H and 50 subjects will receive ABT-378/ritonavir at a dose of 300/75 mg/m2 Q12H. Reverse transcriptase inhibitors therapy will be initiated at the same time as ABT-378/ritonavir. The study duration, based on acceptable safety, tolerability, and antiviral efficacy, will continue in order to allow subjects to receive ABT-378/ritonavir until it becomes commercially available or clinical development is discontinued.